Prevention of tarnish of bronze printing



3,000,751 PREVENTION OF TARNISH OF BRONZE PRINTING This inventionconcerns improvements relatingto the prevention of tarnish on bronze orbrass printing on paper and paperboard materials, whether coated oruncoated, for example bronze printing on folding boxboard. It is to beunderstood that for the purposes of this specification and the appendedclaims the term paper" is to be construed as unlimited as to either thethickness or flexibility of the material and therefore as includingpaperboard. By such printing, various bronze or gold eifects areproduced with metallic pigments. The printing may be carried out byfirst sizing or pre-printing the paper with an undercolour, varnish orother ink medium and subsequently dusting a fine metallic powder,produced from bronze or brass, over the design. Alternatively theprinting may be carried out by using printing inks in which the finemetallic powder is directly incorporated as pigment.

'In the printing industry the term bronze powder is used to denote bothpowders of true bronze and also powders of substances which would not beconsidered true bronze, for instance, brass. 'It is to be understoodthat in this specification the term bronze also includes brass.

The presence of sulphur and sulphur compounds in the atmosphere or asconstituents of paper has a pronounced tarnishing efiect onbronze-printed or brassprinted material. However, acidity of the paperor paperboard is more usually the cause of tarnishing, especially whenthe material is stored in stacks or bundles after the printing.Tarnishing is increased when such printed material is stored inconditions of high relative humidity and temperature, the effect ofrelative humidity being particularly noticeable.

Other factors are also of importance: For example, high copper contentand fine particle size increase the tendency of bronze or brass powdersto tarnish. On the other hand, copper content and particle size alsoinfluence the colour and brilliance of bronze or brass print and somecompromise has frequently to be accepted in practice in regard to thesetwo factors, although very fine powders of high copper content shouldpreferably be avoided.

The invention seeks to prevent or inhibit the tarnishing of suchprinting, particularly where the printed material is stored in the formof stacked sheets, collapsed folded containers or assembled containers,especially in atmospheres of high relative humidity and temperature.

Hitherto, no specific precautions have apparently been taken to preventsuch tarnishing as may occur either during storage or after fabrication.Printings produced by the dusting process, particularly gold eifectsproduced from brass or bronze powders by the usual processes, areespecially liable to tarnishing in atmospheres of high relative humidityand temperature. Even in the case of printings produced with a printingmedium, for example a plastic resin, in which the metallic powder isincorporated, although the said medium offers some protection againsttarnishing, nevertheless tarnishing will occur under unfavourableconditions such as have been referred to above.

According to the present invention, the paper or 3,000,151 esta s aw-s1.

. 2 fabrication, but prior to printing, with a borate of sodium orpotassium, for example sodium metaborate or sodium pyroborate (borax),the amount of anhydrous borate applied being between 2% and 6% by weightby the said material. .1

1 The process may be represented diagrammatically as follows:

paperboard material is treated during fabrication or after Paperorpaper-board material I Treatment oi material with metaor pyro-borate 0!sodium or potassium I Drying oltreated material l Printing of materialwith ink medium and bronze powder Depending upon the material treatedthe borate may be applied by impregnation with or deposition from anaqueous solution or by coating with a coating composition. In the caseof uncoated paper it may be necessary to treat both sides of thematerial, but with paper coated on one side a suflicient degree ofprotection may possibly be achieved by treating the uncoated side only.

The equivalent rate of application of the borate in terms of grammes persquare metre of the surface of the material for any specific rate ofapplication on a percentage by weight basis will depend upon whether thematerial is treated on one or both sides and upon the substance weightof the material so treated.

By way of example, paper of substance weight lbs. per 1,000 sheets each30" x 40" might be treated by impregnation at the rate of 3% ofanhydrous borate, i.e. 5.4 lb. of anhydrous borate per 180 lb. of thesaid paper. Again, for example, folding boxboard of substance weight 510lbs. per 1,000 sheets 30" x 40 coated on one side might be treated bydeposition from an aqueous solution with 2% by Weight of anhydrousborate, i.e. 10.2 lb. of anhydrous borate for 510 lbs. of the saidboard, either by deposition on both sides of each of the 1,000 sheets orby deposition on the uncoated side only or part might be deposited onthe uncoated side and the remainder incorporated in the coatingcomposition.

In the case of uncoated paper it may be necessary to treat both sides ofthe material, but with paper coated on one side, a suflicient degree ofprotection may be achieved by treating the uncoated side only.Nevertheless, if desired, the coated side can be treated either bydeposition or by incorporation of the borate in the coating itself.

Borate solution may be applied in various ways, for example by allowingthe paper to pass between rubber rollers set to transfer the solutionfrom a contained or trough, by spraying the solution on to the paperprior to drying the calendering, by impregnating the paper as it passesthrough a trough containing the solution and sub sequently drying, or byincorporating the borate in a surface coating treatment to which thepaper material is submitted during its manufacture, for example prior tolaquer coating or cast-film coating.

The printing itself may be carried out in any known manner, particularlyin one of the ways referred to above.

We claim:

1. The method of printing on paper with bronze powder which comprisestreating the paper with a composition consisting essentially of asubstance selected from the group consisting of metahorate of sodium andmetaborate of potassium, the amount of anhydrous metaborate appliedbeing between 2% and 6% by weight of the paper and ptinting on thetreated papef with said powder.

2. The method of printing on paper with bronze powder which comprisestreating the paper with a composition consisting essentially of asubstance selected from the group consisting of pyro'borate of sodiumand pyroborate of potassium, the amount of anhydrous pyroborate appliedbeing between 2% and 6% by weight of the "paper and printing on thetreated paper with said powder.

References Cited inthe file of this patent UNITED STATES {PATENTS372,861 Morgan et a1 Nov. 8, 1887 1,565,043 Avis Dec. 8, 1925 1,572,180Buzza Feb. 9, 1926 10 1,964,808 Bottoms July 3, 1934 Wrotnowski Mar.29,1960

1. THE METHOD OF PRINTING ON PAPER WITH BRONZE POWDER WHICH COMPRISESTREATING THE PAPER WITH A COMPOSITION CONSISTING ESSENTIALLY OF ASUBSTANCE SELECTED FROM THE GROUP CONSISTING OF METABORATE OF SODIUM ANDMETABORATE OF POTASSIUM, THE AMOUNT OF ANHYDROUS METABORATE APPLIEDBEING BETWEEN 2% AND 6% BY WEIGHT OF THE PAPER AND PRINTING ON THETREATED PAPER WITH SAID POWDER.